Design and Evaluation of a Distributed Optical Fiber Sensor For Hazard Detection in Mining

Abstract

Mining of precious metals and construction rocks has grown tremendously in Kenya and across the globe. It is a significant source of employment for Kenyans, playing a key role in reducing poverty and dependency rates. Although it acts as a source of livelihood to many, it poses high risks including deaths owing to mine collapse, inhalation of poisonous gases and rock bursts among others. This study aims at developing a fiber optical sensor (FOS) for strain, stress and tensional force measurement. This was achieved by experimental work. A laboratory test-bed was built where by the Fiber Under Test was subjected to strain/stress/tension as power and polarization were monitored. The transmission performance of a distributed feedback (DFB), wave division multiplexing (WDM) and Mach-Zehnder modulator (MZM) laser sources over various fiber links was investigated to enhance maximum coverage of the risk potential area. A 10 Gbps pseudo random bit stream (PBRS) error free transmission of 75 km was achieved on an MZM source while the DFB and WDM source managed to transmit up to 50 km. The effect of mechanical disturbance/stress on power and states of polarization (SOPs) of a signal transmitted through an optical fiber was investigated both in a linear and coiled fiber orientation. Four fiber types namely; G652 Corning, G655 Corning, G655 OFS and PMF Corning were considered. Power fluctuations and SOPs were monitored using the power meter and polarization analyzer respectively when a fiber section was subjected to mechanical stress. It was noted that as loading was done gradually there was a significant decrease in power. The power loss was established after loading the fibers with 18 kg at different times so as to establish their sensitivities kg-1

. The achieved states of polarization were analyzed offline by digital signal processing (DSP) so as to establish the SOP speeds. This was enabled by the use of MATLAB software. The optimal laser-fiber combination for the design was MZM and G655 OFS, with a strain sensitivity of 0.024 dB/kg and SOP speed of 7.29 0

/s. To trigger the alarm, the threshold values were set for the designed sensor. The thresholds for power change and polarization fluctuation were set at 0.2 dB and 1000 respectively. It is through this automation that the designed FOS is expected to give an early alert of the change in soil profile of mines, enabling communication between the miners and the monitors. This will facilitate appropriate emergency response and safety of miners.